Thorium is an element that occurs naturally in the earth's crust.  The only naturally occurring isotope of thorium is 232Th and it is unstable and radioactive.  According to Adams and Gasparini (1970) thorium is chemically stable in the tetravalent ion state under reducing conditions, has a large ionic radii, a high coordination number (8) with respect to oxygen, and complete outermost electron shells.  Because of its large atomic size, high valence and electronegativity, thorium cannot form isomorphic series that involve major rock forming minerals and occurs mostly in accessory minerals.  Under oxidizing conditions, thorium remains stable in the tetravalent state and is not soluble in aqueous solutions.  Some of the accessory minerals that often contain thorium are apatite, sphene, zircon, allanite, monazite, pyrochlore, thorite, and xenotime.  Definitions for the various minerals were taken from the second edition of the Glossary of Geology (Bates and Jackson, 1980).


The radioactive decay series of 232Th is complex and produces alpha, beta, and gamma radiation.  The figure below shows the important isotopes in the decay series, indicates whether the primary decay mode is via alpha or beta emission, and gives the half-life.

Image showing the decay series of thorium-232.


In addition to the alpha or beta particles emitted as a result of the decay of a parent isotope, most of the daughter isotopes also emit gamma rays. The gamma rays have energies ranging from near zero to greater than 2500 keV but most have very low intensities and cannot be used for gamma-ray spectrometry with NaI(Tl) detectors. 208Tl does produce gamma rays with adequate intensities and the table below lists the most intense of those gamma rays. Click here to see a graph showing all of the gamma rays.


208Tl gamma rays

Gamma-ray energy (MeV)

Relative Intensity (percent)















Most gamma-ray spectrometers used for geologic applications measure the intensity of the 2.614-MeV gamma-rays.



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